The present invention relates to a novel stiffening material having melt-adhesive properties, particularly for shoe parts, and to a method for producing such a material.
Thermoplastic stiffening materials are known; they have been used in the shoe industry for a long time. Advisably they are produced in sheet forms and are used in pieces cut therefrom to stiffen the heel region and possibly also the toe region of shoes. The cut pieces can be deformed under the influence of heat, with the stiffening material having to have such properties that, after deformation and cooling, the molded piece will assuredly retain its shape and have good flexibility.
Moreover, the stiffening materials may be coated on one or both sides with thermoplastic adhesives to that during or after deformation they may also be heat sealed to the upper and/or lining material of the shoe.
As is known, thermoplastic stiffening materials may be produced in various ways, e.g. by spreading the coating materials in the form of a dispersion, solution or melt onto woven or knitted or nonwoven textile fabrics. Stiffening sheets may also be produced by laminating thin sheets onto fabric substrates, and most recently also by extrusion of suitable thermoplastic materials.
German Pat. No. 2,621,195 discloses stiffening materials which are produced by melting powdered mixtures of plastics (binders) and fillers onto woven or nonwoven textile fabrics. It is significant for the production process that the powdered fillers are present in a particle size distribution which is comparable to the particle size distribution of the plastic powder, with the particle size distributions in each case lying between 50 and 500 microns, preferably between 100 and 400 microns. If these parameters are maintained, the filler may be used in quantities up to 100 volume percent with respect to the volume of the plastic powder, without this making the melting together of the plastic particles more difficult or impossible.
Although stiffening materials according to German Pat. No. 2,621,195 can still be deformed in a temperature range from 50.degree. to 80.degree. C., they cannot be heat sealed to the upper or lining material without first being provided on one or both sides with an adhesive layer, for example a hot melting adhesive, which today is considered to be a drawback.